There has been proposed, as shown in FIG. 21, an electromagnetic switching device 1000 including a sealed contact device, which is disclosed in Japanese Patent No. 3107288 (corresponding to Japanese Unexamined Patent Publication No. 9-259728). The device 1000 is constructed such that a relay contact portion is housed in a hermetically sealed or air-tight space. With this arrangement, since there is no likelihood that arc generated in opening the contacts may leaked out of the device, this arrangement obviates a space for escaping arc which has been necessary in an air switching device, and makes it possible to mount parts in close contact with the device to raise the packaging density. The device 1000 is constructed as follows.
Referring to FIG. 21, the electromagnetic switching device 1000 comprises a sealed contact portion A, a driving section B, and a housing C. First, the sealed contact portion A is described. The sealed contact portion A comprises a box-like sealing vessel 1 which is made of a heat resisting material such as ceramics with an opening formed in one side thereof. The sealing vessel 1 is formed with two through holes 1a, 1a in the bottom surface thereof. Fixed terminals 2, 2, which are partially received in the through-holes 1a, 1a, each has a substantially multi-layered cylindrical shape made of e.g. a copper material with a closed bottom. A fixed contact 2a is fixedly connected to the closed bottom of the fixed terminal 2, and a flange portion 2c is formed at the axially other end of the fixed terminal 2. The other end of the fixed terminal 2 is opened. The fixed terminal 2 is air-tightly jointed to the sealing vessel 1 around the flange portion 2c by way of brazing or its equivalent in a state that the other end of the fixed terminal 2 protrudes from the sealing vessel 1. An axially-downwardly-oriented screw groove 2b is formed in the other end of the fixed terminal 2 in the open end.
A movable contact piece 3 has a planar shape made of e.g. a copper material. Movable contacts 3a, 3a are respectively fixedly attached at the longitudinal opposite ends of the movable contact piece 3 with a certain distance away from each other in such a manner that the movable contacts 3a, 3a are moved toward and away from the corresponding fixed contact 2a, 2a. An insertion hole 3b is formed in a central part of the movable contact piece 3. A substantially round bar-shaped movable shaft 4 has one end 4a thereof which is received in the insertion hole 3b and the other end 4b thereof which is formed with a screw groove 4c. 
A contact piece holder 5 has a substantially U-shape in cross section and has a bottom wall 5a and a pair of side walls 5b, 5b opposing to each other. The contact piece holder 5 is adapted to hold a compression spring 6 in a compressed and suspended state therein in such a manner that the movable contact piece 3 is operatively linked to the movable shaft 4. A state as to how the compression spring 6 is compressed and suspended in the contact piece holder 5 is described. The bottom wall 5a is formed with an insertion hole 5c in a center thereof through which the one end 4a of the movable shaft 4 is received. The side wall 5b (5b) has an extension 5d (5d) which extends from the middle of a lateral end portion thereof in such a direction as to make the extensions 5d, 5d dose to each other. The extension 5d (5d) is formed with a downward extension (not shown) which extends from the distal end thereof downwardly toward the bottom wall 5a. The contact piece holder 5 is constructed such that the respective outer surfaces of the side walls 5b, 5b oppose to the respective inner surfaces of the sealing vessel 1. A pair of round protrusions 5g, 5g are formed at the outer surface of the side wall 5b (5b). Each protrusion 5g has a thickness substantially equal to the clearance defined by the outer surface of the side wall 5b and the opposing inner surface of the sealing vessel 1. The compression coil spring 6 is adapted to urge the movable contact piece 3 in such a direction as to render the movable contacts 3a, 3a in abuttal contact with the fixed contacts 2a, 2a. Thus, the compression spring 6 is retained in the contact piece holder 5.
A fixed iron core 7 has a generally cylindrical shape with one end 7a thereof having a larger diameter than that of the primary part thereof. An insertion hole 7b is axially formed in the fixed iron core 7 for receiving the movable shaft 4 therein. The fixed iron core 7 is fixedly connected with a first joint member 11 at the one end 7a by insertion of the fixed iron core 7 into a through-hole 11a of the first joint member 11. The fixed iron core 7 has a recess 7c at the other end thereof having the inner diameter larger than the inner diameter of the insertion hole 7b. 
A generally cylindrical-shaped movable iron core 8 is formed with an axially extending insertion hole 8a through which the movable shaft 4 is inserted. The movable iron core 8 is formed with a screw grove 8b along the axial direction thereof to desirably shift the coupling position of the movable shaft 4 and the movable iron core 8 along the axial direction of the movable shaft 4 in cooperation with the screw groove 4c of the movable shaft 4. The movable iron core 8 has an opposing portion 8c at an axially end thereof opposing to the fixed iron core 7, and a recess 8d at the axially other end thereof having the inner diameter larger than the inner diameter of the screw groove 8b. The outer surface of the movable iron core 8 constitutes a sliding surface 8e which is rendered in sliding contact with the inner circumference of a cylindrical member 10 having a closed bottom, which will be described later.
A return spring 9 is adapted to urge the movable iron core 8 in such a direction as to move the movable contacts 3a, 3a away from the fixed contacts 2a, 2a. The return spring 9 is in the form of a coil and has the inner diameter slightly larger than the inner diameter of the insertion hole 7b of the fixed iron core 7. When the movable shaft 4 is inserted in the insertion hole 7b of the fixed iron core 7, and one end of the return spring 9 is fitted in the recess 7c of the fixed iron core 7, the return spring 9 is positioned relative to the recess 7c. 
The cylindrical member 10 has a cylindrical shape made of a non-magnetic material with a closed bottom and includes a main part 10a and a bottom part 10b. The movable iron core 8 is housed in the bottom part 10b, while the fixed iron core 7 is housed in the cylindrical member 10 at the open end with the opposing portion 8c opposing the fixed iron core 7.
The first joint member 11 is made of a magnetic metal material such as iron and has a rectangular shape. The first joint member 11 constitutes a magnet circuit along with the fixed iron core 7 and the movable iron core 8. As mentioned above, the first joint member 11 is formed with the insertion hole 11a in the center thereof for receiving the one end 7a of the fixed iron core 7 prior to its fixation to the first joint member 11. The first joint member 11 is air-tightly connected with the cylindrical member 10 around the insertion hole 11a. 
A second joint member 12 is made of a metallic material and has a cylindrical shape with a hollow 12a formed at the axially opposite ends thereof. The second joint member 12 has a first joint portion 12c at an axially one end thereof to be air-tightly connected with the open end of the sealing vessel 1, and a second joint portion 12b at the axially other end thereof to be air-tightly connected with the first joint member 11. The second joint member 12 is formed with a stepped portion 12d around its circumference at an appropriate position of the cylindrical part thereof. By forming the stepped portion 12d, the cross-section of the hollow 12a has a larger diameter at a portion between the first joint portion 12c and the second joint portion 12b. A sealed space 30 is defined by air-tightly connecting the second joint member 12, the sealing vessel 1, and the first joint member 11 each other to accommodate the fixed contacts 2a, 2a, the movable contacts 3a, 3a, the fixed iron core 7, and the movable iron core 8 therein. The sealed space 30 is hermetically sealed with hydrogen gas or gas containing hydrogen as a primary component of e.g. about 2 atmospheric pressure contained therein.
Next, the compressed and suspended state of the compression spring 6 is described. First, the movable contact piece 3 is fitted in the contact piece holder 5 in a state that the movable contacts 3a, 3a faces the insertion hole 5c. Next, the compression spring 6 is fitted in the contact piece holder 5 in a certain compressed state. Specifically, the compression spring 6 is suspended on the extensions 5d, 5d in a state that one end thereof is connected with the bottom wall 5a of the contact piece holder 5 by way of the movable contact piece 3, and the other end thereof is engaged with the downward extension (not shown) of the contact piece holder 5. More specifically, the bottom wall 5a of the contact piece holder 5 constitutes a first suspending portion for suspending the one end of the compression spring 6 by way of the movable contact piece 3, and the extensions 5d, 5d constitute a second suspending portion for suspending the other end of the compression spring 6. The one end 4a of the movable shaft 4 is received in the compression spring 6 and in the insertion hole 3b of the movable contact piece 3, and then inserted into the insertion hole 5c of the movable terminal 5 to thereby fixedly hold the movable shaft 4 in the holder 5 around the insertion hole 5c. 
The switching device 1000 further comprises magnetic means (not shown) including a permanent magnet and a pair of magnetic members with the permanent magnet provided therebetween. The magnetic members are attached to the respective corresponding outer surfaces of the sealing vessel 1 in such a manner that the magnetic members sandwich the fixed contacts 2a, 2a and the movable contacts 3a, 3a therebetween. The magnetic means generates a magnetic field in the space where the contacts 2a, 2a are disposed in a direction orthogonal to the moving direction of the movable contacts 3a, 3a. 
Next, the driving section B is described. The driving section B constitutes a magnet device along with the fixed iron core 7, the movable iron core 8, and the first joint member 11. A coil 13 is wound around a coil bobbin (coil frame) 14. A yoke (iron joint) 15 includes a yoke main body 15a and a bush 15b. The yoke 15 constitutes a magnet circuit along with the fixed iron core 7, the movable iron core 8, and the first joint member 11. The yoke main body 15a has a generally U-shape such that a bottom wall and a pair of opposing side walls encase the coil 13 therein. The bottom wall of the yoke main body 15a is formed with a through-hole 15c in the center thereof. The bush 15b has a cylindrical shape, and is fitted in the through-hole 15c of the yoke main body 15a. The cylindrical part 10a of the cylindrical member 10 is disposed between the bush 15b of the yoke 15 and the movable iron core 8 in a state that the bush 15b is fitted in the through-hole 15c of the yoke main body 15a. 
Lastly, the housing C is described. The housing C is adapted to accommodate the sealed contact portion A and the driving section B therein. The housing C is formed with an insertion hole 16 for receiving the fixed terminal 2. The flange portion 2c of the fixed terminal 2, when received in the insertion hole 16, protrudes out of the housing C. The protruding part of the fixed terminal 2 is connected with a terminal plate (not shown) for connecting an electric wire.
Now, described are operations as to how the fixed terminals 2, 2, and the movable contact piece 3 are electrically communicable and discommunicable each other in the thus constructed electromagnetic switching device 1000 including a sealed contact device in response to an input signal.
Before energizing the coil 13, the movable contact 3a (3a) is opposed to the corresponding fixed contact 2a (2a) with a certain gap L1. When the coil 13 is energized in response to input of an operative signal to the electromagnetic switching device 1000, the movable iron core 8 is magnetically attracted to the fixed iron core 7 and is rendered movable. Thereby, the movable shaft 4, which is screwed to the movable iron core 8 and fixed thereto by an adhesive or the like, is driven. As the movable shaft 4 is driven, the gap L1 is gradually decreased with the result that the movable contact 3a (3a) contacts the corresponding fixed contact 2a (2a). Then, a load of the compression spring 6 is sharply raised. As the load of the compression spring 6 is sharply raised, the movable shaft 4 is further driven. As a result, the movable contact 3a (3a) is moved further toward the corresponding fixed contact 2a (2a) by an over-travel distance, thereby further increasing the load of the compression spring 6. The sum of the gap L1 and the over-travel distance corresponds to a stroke of the movable iron core 8.
When input of the operative signal is suspended, and the coil 13 is de-energized, the movable contact piece 3 is returned to its original position primarily by an urging force of the compression spring 6 and the return spring 9 to thereby displace in the direction opposite to the aforementioned direction relative to the fixed contacts 2a, 2a. Thus, the movable contact 3a (3a) is moved away from the corresponding fixed contact 2a (2a). Simultaneously, the movable iron core 8 is returned to its original state by displacing relative to the fixed iron core 7 by a certain distance. Arc developed between the contacts while the movable contact piece 3 and the movable iron core 8 are being returned to their respective original positions sufficiently spreads in the extending direction of the movable contact piece 3 toward the opposite ends thereof by a magnetic field generated by the magnetic means (not shown), whereby the developed arc is wiped out.
In the conventional device 1000, however, since the cylindrical member 10 is made of a non-magnetic material, as shown in FIG. 22, there exists a gap G between the bush 15b and the movable iron core 8, which may cause magnetic loss or lowering of electromagnetic attracting force. Therefore, it is highly likely that switching performance of the device 1000 may be deteriorated owing to increase of the dimensions of the electromagnetic portion of the device 1000 or lowering of a spring load. Further, in the conventional device 1000, it is required to continue applying an operative signal of a relatively large pulse to keep on electrically communicating between the fixed terminals 3, 3 (sic), and the movable contact piece 3. In the case where the device 1000 is used as a relay for power-driven load, it is preferable to suppress power consumption by the coil which is required in applying an operative signal. Further, in the case where the device 1000 is incorporated with a control circuit block on which a control circuit is formed so as to control driving of the contacts, it is preferable to electrically connect electrodes of the control circuit block with the coil with a simplified configuration.
Japanese Unexamined Patent Publication No. 9-259728 proposes a cylindrical member 10 of a three-piece structure including a closed bottom portion, a cylindrical part made of a magnetic material, and a cylindrical part made of a nonmagnetic material in order to overcome the drawback such that switching performance of the device is lowered due to increase of the dimensions of the electromagnetic portion of the device or lowering of the spring load. The arrangement disclosed in the publication can obviate the gap G between the bush 15b and the movable iron core 8. Specifically, as shown by the arrow X in FIG. 23, the attracting force of the electromagnetic portion of the device is raised from point Q to point Q′, and the spring load W is increased as shown by the arrow Y in FIG. 23. Thus, the switching performance of the device can be improved. However, in the latter example where the cylindrical part has a three-piece structure, the number of parts constituting the device is increased, and the production cost is raised notwithstanding the improvement in magnetic efficiency of the electromagnetic portion.
As a measure for suppressing the power consumption by the coil, generally proposed is Pulse Width Modulation (PWM) control. However, in such a control, noise emitted from the coil is relatively large, which may adversely affect electronic parts in the vicinity of the coil and the electromagnetic switching device, etc.